Generally, in preparing liquid crystal display devices, liquid crystal cells comprising liquid crystals and polarizing plates are basically required, and adhesive layers or pressure-sensitive adhesive layers suitable for bonding them have to be used. In addition, for improving functions of liquid crystal display devices, a phase retardation plate, a compensation plate for wide view angle, a brightness enhancement film, and the like may be used, with additionally adhered to the polarizing plate.
Major structure forming a liquid crystal display device comprises, generally, a uniformly aligned liquid crystal layer; a polarizing plate with a multi-layer structure, incorporated into a pressure-sensitive adhesive layer or an adhesive layer, based on a liquid crystal cell consisted of a transparent glass plate or plastic sheet material containing a transparent electrode layer; a phase retardation plate; and an additional functional film layer and the like.
The structure of polarizing plate is one comprising an iodine compound or a dichroic polarizing material aligned in a certain direction. To protect these polarizing elements, multi-layers are formed on both sides using a protective film such as triacetyl cellulose (TAC). In addition, the polarizing plate may additionally comprise a phase retardation film, a compensation film for wide view angle such as a liquid crystal type film, a brightness enhancement film, a reflective layer, or an antiglare layer, in a shape having a unidirectional molecular alignment.
Each film mentioned above is made of materials having different molecular structures and compositions, and so have different physical properties. Especially, under heat or moisture-heat conditions, the dimensional stability according to shrinkage or expansion of materials having a unidirectional molecular alignment is insufficient.
TN (Twisted Nematic) shaped or STN (Super Twisted Nematic) shaped liquid crystal display manner is generally such that polarizing plates are adhered to both surfaces of a liquid crystal panel, with crossing their optical axes of 45 and 135. In said liquid crystal display manner, when the polarizing plate is fixed by a pressure-sensitive adhesive, a deforming stress by heat or moisture-heat environment remains in a residual state and thereby light leakage is caused at portions on which the stress is focused. To solve such problem, a research has been tried, which is intended to improve light leakage by providing a pressure-sensitive adhesive with function of stress relieving and lessening stress concentration with shrinkage of the polarizing plate.
However, in liquid crystal display devices such as IPS (In-Plane Switching), or VA (Vertical Alignment), mainly utilized in large TVs, polarizing plates are adhered to both surfaces of liquid crystal panels with crossing their light axes of 0 and 90. Such liquid crystal display devices cause light leakage by stress concentration in heat or moisture-heat environment, but it is known the bend of liquid crystal panels rather than the stress concentration as a main cause. Said bend of liquid crystal panels is caused by difference (especially, elongation axis direction) in shrinkage ratios of polarizing plates depending on their positions under heat or moisture-heat conditions. In TN or STN liquid crystal display device, that is, polarizing plates are adhered with crossing their light axes of 45 and 135 so that elongation axis directions of upper and lower polarizing plates are positioned evenly or symmetrically, whereas polarizing plates in liquid crystal display devices such as IPS or VA are adhered with crossing their axes of 0 and 90 so that elongation axis directions of upper and lower polarizing plates are asymmetrical. Accordingly, in the liquid crystal display devices such as IPS or VA, the bend caused by the upper polarizing plate is different from that caused by the lower polarizing plate, and whereby the bend of liquid crystal panel is caused in a direction of high bend over all. The liquid crystal panels are pressured by top cases used in fixing them, due to such bend of liquid crystal panels, and whereby the light leakage according to unevenness of liquid crystals is caused.
JP Unexamined Patent Publication No. 2006-58718 discloses a pressure-sensitive adhesive for polarizing plates having a storage elasticity modulus of 105 to 109 Pa at 0 to 50° C. after cross-linking and a glass transition temperature of −20° C. or more, to inhibit the light leakage of liquid crystal display devices such as IPS or MVA manners. It is described in said disclosure that the light leakage can be improved by using pressure-sensitive adhesives with high elasticity modulus and glass transition temperature over conventional pressure-sensitive adhesives. However, it is difficult to improve the light leakage by bend of liquid crystal panels with the method described above, as well as it is not mentioned to improve bend of liquid crystal panels such as IPS or MVA manners.